The present invention relates to flapping restrainer devices for the rotor blades of rotorcrafts.
More specifically, the invention relates to a flapping restrainer device which is intended to equip a main and articulated rotor head of a helicopter, and the flapping restrainer device according to the invention finds a particularly beneficial application in equipping rotor heads comprising integrated hub-masts, especially integrated hub-masts of the type comprising a monobloc tubular body, as described in French Patent Applications Nos. 2,584,995, 2,584,996 and 2,584,997 of the assignee.
In order to limit the downward flexion or angular flapping of the blades of the articulated main rotors of helicopters, under the effect of the blades' own weight, when the rotors are rotating at low speeds or when they are stationary, the rotor heads are generally equipped with flapping restrainers referred to as "lower restrainers", and numerous different constructions of lower restrainer devices have already been proposed.
One of these known constructions is particularly appreciated on account of the advantages which it exhibits, namely a structure which is very simple and inexpensive to manufacture, and has a mode of operation which is entirely satisfactory in service and exhibits a high degree of reliability. In this construction, the flapping restrainer device comprises a lower and central restrainer, which is common to all the blades, and comprising at least one rigid ring, referred to as "reciprocal", which is mounted so as to be displaceable laterally in relation to the rotor mast, and against which there are supported, when the rotor rotates at low speed or is stationary, rigid lower runners, each one of which is integral with a lower part of the root of a blade or with a component for connecting said blade to the hub of the rotor.
When the rotor rotates on the ground at a speed exceeding a given threshold, or in flight, for example when it rotates at its nominal speed, the mounting of each laterally displaceable reciprocal ring permits the inclination of the disc of the rotor by the cyclic control, on account of the fact that the blade which passes at the lowest position, in the course of its rotation about the axis of the rotor, pushes back each reciprocal ring on the opposite side, towards a blade which then passes to the highest position, and thus allows each reciprocal ring the possibility of being displaced laterally, without opposing the movements of the blades.
Such a lower flapping restrainer device incorporating at least one reciprocal ring likewise permits the pitch control of the blades, when the rotor is stationary, in such a manner as to control the free displacement of the pitch control. In fact, in this case, when a change of the pitch of the blades is commanded, the latter are supported on the reciprocal ring or rings by their lower supporting runners and entrain each reciprocal ring over a small rotation with a very slight sliding of the supporting surfaces.
Such lower flapping restrainer devices comprising lower supporting runners which are rigidly integral, in terms of movements, with the blades, and which are supported against a supporting track formed by the periphery of at least one reciprocal ring mounted to slide radially about the rotor mast and below the hub, are described, for example, in U.S. Pat. No. 2,742,098 and in French Patent 2,427,251 of the assignee.
In U.S. Pat. No. 2,742,098, the device for downwardly limiting the flappings of the blades of a rotorcraft main rotor comprises a lower and central restrainer incorporating two reciprocal rings superposed so as to slide freely in a radial plane within an annular and flat casing, concentrically surrounding the rotor mast, with which it is integral just below the hub. The radial movements of the reciprocal rings are limited by a restrainer ring, of smaller diameter, which is accommodated within the casing and within the reciprocal rings, and which can either rotate together with the mast, while presenting an external radial face which is concentric with the mast, or be fixed and eccentric in relation to the axis of the mast, while being supported by a fixed sleeve surrounding the mast, in order to permit a greater angular displacement of the blades towards the front of the rotor than towards the rear of the rotor. The casing accommodating the reciprocal rings exhibits radial slots, in which there engage runners, each one of which is rigidly integral with the components for connecting a blade to the hub, and projecting below these components, in order to come into contact against the external radial face of the reciprocal rings, in such a manner that, when the rotor is stationary or when the latter is rotating at low speeds, the angular flapping displacements of the blades are downwardly limited by the reciprocal rings, then occupying a middle position, precisely concentric with the axis of the rotor.
In the patent FR 2,427,251, which relates to rotors, the hubs of which are formed in a single piece with the corresponding rotor masts, in the form of integrated hub masts, to which the root of each corresponding blade is coupled via, on the one hand, retaining and articulation means comprising at least one laminated bearing or stop preferably spherical, having two armatures, one of which, in the external radial position in relation to the other, is fixed to the hub, and, on the other hand, a forked piece having two radial branches, which is integral with the blade root and the internal radial ends of the two branches of which are fixed to the other armature of the laminated bearing, there has been shown in FIGS. 3 and 5, and described with reference to these figures, a lower flapping restrainer device comprising a single metallic reciprocal ring, sliding radially and freely in a radial slot of a single annular piece, which is fixed about the rotor mast and below the hub, or of a plurality of supporting pieces regularly spaced over the periphery of the mast. This reciprocal ring cooperates with metallic lower restrainers, each one which is in a corner configuration and fixed by a wing against the lower face of the internal radial end of the lower branch of the forked piece for connecting a corresponding blade to the hub, the fixing being ensured by bolts for fixing the branches of this forked piece to the internal armature of the laminated spherical bearing.
However, these lower flapping restrainer devices incorporating a reciprocal ring exhibit a disadvantage when a gust of wind, occurring during the critical phases of start-up or of stopping of the rotor, when the speed of rotation of the latter is less than a given threshold, and therefore when the centrifugal force is not yet sufficiently high to stabilize the blades in their plane of rotation, pushes back one of the blades strongly in a downward direction, in such a manner that this blade is violently pressed onto the reciprocal ring which, not being subjected to sufficient reaction forces on the part of the other blades, adopts an extreme position, in which it does not prevent the blade which has been violently pushed back in a downward direction from reaching a dangerous position, of high inclination, and from possibly striking or even severing, by its end, the tail beam or the cabin of the helicopter. The same applies, when the rotor is stationary or when the latter is rotating only at low speed, when a blade is urged in an upward direction by a strong gust of wind. In this case, this blade which is thus urged may adopt a high angle of conicity, and then fall back heavily onto the reciprocal ring, possibly causing damage to this ring or to the supporting runner integral with the blade, and itself undergoing an instantaneous flexional torque which is far greater than the loads which it may normally encounter; this may lead to its being put out of service.
In order to remedy this disadvantage, without nevertheless renouncing the use of a common lower restrainer of the type incorporating a reciprocal ring, the advantages of which have been emphasized, it has already been proposed, especially by French Patents 948,570 and 2,434,079, to construct a flapping restrainer device combining lower restrainers incorporating a reciprocal ring and a lower runner for supporting the blades, and upper restrainers, limiting the angular displacements of the blades in an upward direction.
In this case, in fact, when a blade is violently urged angularly in a downward direction by a gust of wind when the rotor is stationary or rotating at low speed, the lower restrainer or lower supporting runner of this blade radially pushes back the reciprocal ring, which itself tends to push back the lower supporting runner of at least one of the other blades of the rotor, in the sense of the lifting of this other blade or of these other blades. However, this lifting is limited by the upper flapping restrainers which, when the rotor is stationary or rotates at low speed, are always in the working position. Thus, such a flapping restrainer device combines lower restrainers and upper restrainers which cooperate with one another in such a manner as to reduce the risks of damage to the rotor, when it is subjected to violent gusts of wind, while it is driven at a speed of rotation below a given threshold.
In the patent FR 948,570, the reciprocal ring is mounted so as to be resiliently laterally displaceable by a rubber ring or a plurality of rubber sleeves or blocks distributed about it, or alternatively by springs, which connect an upper face of the reciprocal ring, which is inclined with respect to the axis of the rotor pylon, to a lower face which is likewise inclined, of a crown integral with the rotor pylon, about the latter and just below the hub. In order to limit the angular displacement of the blades in a downward direction, fingers projecting below the blades are supported against the external peripheral surface of the reciprocal ring, which peripheral surface is preferably disposed at a radial distance which is as large as possible in relation to the axis of the rotor. The upper restrainers likewise comprise fingers projecting above the blades and directed radially towards the axis of the rotor, to come into contact against a fixed restrainer on the upper part of the hub, in order to limit the flappings of the blades in an upward direction.
Accordingly, the lower restrainers comprise a rigid reciprocal ring, which is resiliently displaceable, and which is thus restored towards its position concentric with the rotor axis, which cooperates with fixed lower supporting fingers of the blades, and the upper restrainers comprise restrainers which are situated at the top and fixed on the hub and which cooperate with upper and fixed supporting fingers of the blades.
In the patent FR 2,434,079 of the assignee, the reciprocal ring, which has a cross-section in the form of a reclining T, is mounted so as to slide radially by its thin internal part engaged into the forks presented at their lower part by three calipers fixed by an upper elbowed element about the rotor mast and below the hub. A part made from synthetic material resistant to wear and to shocks is screwed against the internal face of the lower end of each caliper, to be thus inserted between the latter and the external radial face of the rotor mast, and one end of a spring blade is gripped between this part and the caliper which carries it, this spring blade being curved towards its other end in order to urge the internal edge of the reciprocal ring, in such a manner as to tend to maintain the latter in a mean position concentric with the axis of the rotor mast.
The lower supporting runner of each blade is a rigid piece, having a cross section in the form of a T, which is fixed by a wing against the lower face of the lower branch of the forked piece for connecting the blade to the hub, by bolts for fixing the internal radial ends of the two branches of this forked piece to an internal armature of a laminated bearing for the retention and articulation of this blade on the hub, and the substantially vertical face of the supporting runner which is opposite the reciprocal ring, and which is intended to cooperate with it, is covered with a covering in the form of a thin layer of a material having high resistance to shocks and to wear.
The upper restrainers comprise, for each blade, on the one hand an upper supporting runner, fixed against the upper face of the upper branch of the forked piece for connecting the blade to the hub, by bolts for fixing the internal radial ends of the two branches of this forked piece onto the internal armature of the laminated bearing for the retention and articulation of this blade on the hub and, on the other hand, a restrainer which is situated at the top and which is carried by the hub and can be retracted or drawn in by the action of the centrifugal force, to which it is subjected when the rotor rotates with a speed of rotation greater than a given threshold, which is sufficient to permit the control of the rotor by the flight commands of the rotorcraft.
Numerous different constructions of individual flapping restrainers for each blade and automatically retractable by the effect of the centrifugal force, against restoring springs which tend to bring them back into a working position, in which they limit the angular displacements of the blades on one side of their plane of rotation, have already been proposed to constitute lower restrainers as well as upper restrainers (see, for example, the patent FR 1,368,684). Each one of such restrainers most frequently comprises a pivoting lever, urged by a restoring spring, and a first arm of which constitutes a supporting restrainer per se, while a second arm carries a governor weight sensitive to the action of the centrifugal force, to cause pivoting of the lever against the restoring spring.
In the patent FR 2,434,079, the lever is elbowed and mounted to oscillate with radial play on a pivot, having an axis parallel to the axis of rotation of the rotor, and fixed, in the vicinity of the axial plane of symmetry of the corresponding blade at rest, on a rigid crown carried by the upper part of the hub, the spring maintaining the lever, when the rotor is stationary or rotating at low speeds of rotation, in such a manner that its first arm is in a position of stopping of the upper supporting runner of the corresponding blade, and that its second arm is supported, over a part of its length, against the crown supporting the pivot. The two arms of the elbowed lever form an angle such that the centrifugal force applied to the governor weight, when the rotor rotates, tends to overcome the restoring force of the spring and to cause pivoting of the lever in a direction such that its first arm is remote from the Opposite upper supporting runner on the corresponding blade. A ring of synthetic material having high resistance is mounted onto each pivot, between the latter and the corresponding pivoting lever, with which the ring exhibits a radial play sufficient for the shocks and forces transmitted by the upper supporting runner to the first arm of the pivoting lever to be directly transmitted to the crown by the second arm of this lever, and not to the pivot, in such a manner as to protect the latter from these shocks and forces.
In a lower flapping restrainer device comprising a reciprocal ring surrounding the rotor mast, having regard to the high rigidity which this reciprocal ring must exhibit, its weight is acceptable only if its diameter is relatively small; this is generally the case in the rotors of the prior art presented hereinabove, which are equipped with a tubular rotor mast of a fairly small diameter.
However, it has recently been proposed to equip modern helicopters with main rotors on which the mast and the hub are constructed in the form of an integrated tubular hub mast assembly. This has led the designers, for reasons of mass optimization of this hub mast assembly, to reduce the thickness of the mast and simultaneously to increase its diameter, in such a manner that the mounting of a lower restrainer reciprocal ring about such a hub mast leads to a prohibitive weight for this reciprocal ring.
For this reason, lower restrainer devices incorporating a reciprocal ring are mounted within integrated tubular hub masts, such as those described in its French Patents 2,584,995 and 2,584,996. These lower restrainer devices are, in particular, represented in FIGS. 1, 2, 5 and 6 of the patent FR 2,584,995 and in FIGS. 4 and 6 of the patent FR 2,584,996, and have described with reference to these Figures.
They comprise a reciprocal ring constructed of composite material surrounded by a band constructed of cemented steel, which is mounted radially and freely slidingly, within the hub mast, in an annular guide having a cross-section in the form of a reclining U and open radially towards the exterior, this guide being maintained coaxially in the hub mast by a support, which is likewise coaxial with the hub mast and essentially accommodated in the latter.
In the case where the hub mast is surmounted by a shroud, the support of the lower flapping restrainer which is central and common to the blades and which is formed by the reciprocal ring and its annular guide may be a support which is upwardly flared, carrying the central flapping restrainer by its narrowed lower part, while it is fixed to the shroud by its flared end. In the absence of a shroud, this support may likewise comprise a frustoconical portion internal to the hub mast and carrying the lower and central flapping restrainer by its minor base, while its major base is extended by a portion which is flared and turned down outside the hub mast, above the free upper edge of the latter, to which the support is fixed by screws. However, with or without a shroud on the rotor mast, it is likewise possible that the support of the central flapping restrainer comprises a substantially frustoconical tubular bearing surface for support against the internal face of the mast, and a tubular centering shell, likewise substantially frustoconical, which is integral with the bearing surface and which supports the central flapping restrainer.
In order to cooperate with the reciprocal ring internal to the hub mast, and for each blade, a rigid supporting runner is fixed against the lower face of the internal radial end of the lower branch of a forked piece for connecting the blade to the hub, by bolts for fixing the internal radial ends of the two branches of this forked piece to the internal armature of a spherical laminated bearing for the retention and articulation of the corresponding blade on the hub and, as on this type of integrated tubular hub mast, the lower branch of this forked piece traverses an opening formed in the wall of the hub mast, and that at least the lower part of the laminated spherical bearing and of its internal armature is likewise accommodated in the hub mast, the supporting runner is itself accommodated in the latter.
Thus, on account of the arrangement of the reciprocal ring within the hub mast, the diameter and therefore the mass of the reciprocal ring are less than they would be if this reciprocal ring were mounted about the hub mast.
However, on account of the fact that the lower supporting runners, which are integral in terms of movement with the blades are likewise accommodated within the hub mast, this gives rise to major disadvantages regarding the construction and the utilization of such hub masts.
In fact, as the mounting of these runners below the internal ends of the lower branches of the forked pieces for connecting the blades to the hub is carried out at the same time as the internal ends of the two branches of these forked pieces are fixed to the internal armatures of the laminated bearings, after introduction, on the one hand, of at least the lower branches of these forked pieces into corresponding and lateral openings of the hub mast and, on the other hand, of the laminated bearing into the hub mast through the free opening at the upper end of the latter, it is necessary, for reasons of accessibility to the runners, to provide in the wall of the hub mast openings for passage of said lower branches which are of a size very much greater than that which is sufficient to permit, in service, the displacements of the blades about their pitch, flapping and drag axes. However, the formation of openings of such a size in the hub mast results in causing fragility of the latter to a considerable extent. Furthermore, in order to proceed with the maintenance and servicing operations at the location of the reciprocal ring and of the supporting runners of the blades, in order to replace, for example, the wear components mounted on these elements or these elements themselves, the accessibility through the opening of the upper end of the hub mast is insufficient on account of the fact that these elements are disposed below the laminated bearings and the portions of the connecting branches fixed to the latter, and on account of the bulkiness of these components within the upper part of the hub mast, which constitutes the tubular hub body.
Furthermore, for the practical construction of such a lower restrainer by reciprocal ring internal to the mast, it is difficult, and in certain cases impossible, to accommodate the rigid supporting runner integral with the forked piece for connecting the blade to the hub, in such a manner that it does not come to be restrained on the internal wall of the mast in the course of the maximum upward angular displacement of the blade and thus of the forked piece for blade-hub connection, especially in the case where the internal diameter of the frustoconical mast diminishes rapidly in the direction from the mast root.
These displacements of the blades are, in fact, in the order of 27.degree., with respectively +20.degree. upwards and -7.degree. downwards.
Moreover, the lower restrainer components of the blades must accept the recovery of the supporting torque of the blades, which is equal to the product of the weight of the blade (from 30 to 100 kg for helicopters of medium tonnage) and the distance from the center of gravity of the blade to its center of pivoting on the hub (a distance amounting to 5 to 9 meters for medium-sized helicopters).
This supporting torque, which is already naturally high, must be multiplied by an increasing coefficient of 2 to 3 in order to take account of the most unfavourable case of loading, which is that of the rolling on the ground over bumpy terrain, of a towed helicopter with the rotor stationary.
This large supporting torque, with the rotor stationary, is absorbed by the lower restrainer device which, in order not to be urged by prohibitive forces, demands the provision, between the center of pivoting of the blade and the point of support of the lower restrainer, of a lever arm having the greatest possible length.
However, if this lever arm is increased, there is also an increase in the space required for the displacements of the mechanism at the extreme angles and, consequently, in the bulkiness of the lower restrainer element connected to the sleeve.
Given, on the one hand, the need to over-dimension the thickness of the mast in the zone where the passage holes for the lower element of the forked piece for the blade-hub connection are formed and, on the other hand, the requirements for free displacement of the supporting runner connected to this forked piece, it is very difficult to accommodate within the mast all the components of the lower restrainer mechanism having a reciprocal ring.
In order to limit the displacements of the blades on one side and/or on the other side of the plane of rotation of the blades of a rotor incorporating an integrated tubular hub mast the patent FR 2,584,997 of the assignee likewise discloses a flapping restrainer device which comprises a central restrainer, fixed coaxially in the hub mast and carrying at least one restrainer surface axially bordering on a concave bearing surface, and limiting the displacements of the internal radial armatures of the laminated bearings by contact with lugs projecting radially towards the interior on these armatures, on one side and/or on the other side of a convex bearing surface machined in the base of recesses formed in the internal radial faces of these armatures, and sliding against the concave bearing surface of the central restrainer.
However, such a flapping restrainer device, although suitable for the integrated hub mast of a helicopter rear rotor, the plane of rotation of which is substantially vertical, is not appropriate for equipping a hub mast of this type, for a main rotor, the plane of rotation of which is substantially horizontal. In fact, the moment of embedding of the blades of a main rotor in the hub, this being a moment resulting from the weight of the blade multiplied by the distance of its center of gravity from the flapping articulation, being very high, cannot be accepted by this type of flapping restrainer, essentially by reason of the small constant lever arm between the supporting lugs and the center of rotation of the laminated bearings.